1. Geologic and stratigraphical setting

Petrographic and mineralogical characterization of Middle Jurassic thrombolites (High Atlas, Morocco)

¹ University of Potsdam, Germany ² Ruhr-University Bochum, Germany Contact: martin.homann@gmail.com

M. HOMANN¹, S. TOMÁS¹, F. AMOUR¹, M. ZITZMANN¹, M. MUTTI¹, N. CHRIST², A. IMMENHAUSER²

SEPM-CES-GVSediment 2010 meeting, PotsdamGermany, June 25th - 27th, 2010

3. Petrography and composition

Mud-marl alternance

Marl

Bioclastic peloidal Wkst

Bioclastic Fst (gastro-coral)

Peloidal ooidal Grst

Ooidal Grst

Bioconstructions

Bioclastic Fst (echinoid-oyster)

Gastropods

Bivalves (undiff.)

Oysters

Intraclasts

Cross-bedding

Burrows

Bioturbation

Echinoderms

Brachiopods

Foramns

Peloids

Oncoids

Ooids

Coral

Bioclasts (undiff.)

Sponges

Bacterial activity

Microbialite

Debris

Coral branches

massiv coral

V Evap. (pseudomorphs)

CS Condensed surface

4. Mineral structure 5. Discussion and conclusion

MOROCCO

Amellago Canyon

Rich

Tanger

Marrakesh

100 km

Rabat

36°

34°

32°

30°

350° 352° 354° 356° 358°

References

3.1 Macrostructure 3.2 Mesostructure

3.3 Microstructure

1.3

m

3.1 m

G1

G2

E-Fst. MM14

S N

a

1.8

2.41

1.1

(m)

1.4

G2

G1

MM8

MM9E-Fst.

b

G1 G1

E-Fst

G1E-Fst

G2

G2

G2

MM

Mound

a b

c

d e

2. Microbialite structure

NS

E-IW N-IF

70 m150 m

90 m

S1S1

0 mP

W GF

RBM

m

cs

E-IW

V

V

18.5 m

G1

G2

0 mP

W GF

RBM

m

N-IF13.5 m

?

cs

G1

G2Mounds Mounds

Domal growth morphology Hemispheroids between two domes

E-IW N-IFheight 0.5 - 2.2 (ø1.2) 0.7-3.2 (ø1.4)width 1.2 - 3.3 (ø2.5) 1.3 - 4.7 (ø2.45)

d

The studied Middle Jurassic microbial mounds crop out in the Amellago Canyon, located in the central part of the High Atlas Mountain range of Morocco. During the Bajocian shallow-water carbonate-terrigenous mixed sediments accumulated along a gently steeping ramp. The studied succession, up to 100 m thick, belongs to the Assoul Fm and mainly consists of lagoonal facies, ooid shoals and bioconstructions.

The thrombolites are composed of dark mesoclots (40 - 70%), which are characteristic for their distinct, clotted fabric (a, e). These polymor-phic clots occur in clusters and interconnect with each other to form aggregated assemblages of micrite. In the interspace between the mesoclots irregular shaped growth framework cavities were devel-oped. They make up to 10 - 30% of the thrombolite total volume and are usually flat and elongated with a height of 3 mm and a length of 5 mm. The cavities can be partially or totally filled with fine-grained sedi-ment and calcite. Geopetal fillings have been observed (a,b). Also do-lomitization occurs inside some cavities (c). No allochthonous par-ticles or cryptic biota are present inside the cavities. Skeletal components also occur inside the thrombolite fabric, e.g. bryozoans, small branching corals and sponges. The thrombolites often have horizontal and vertical borings (f), which are indicators of their early lithification.

The microbial mounds (c) appear embedded in a 4-m-thick package composed of peloidal-ooidal packstones and grainstones. This pack-age can be subdivided into two parts, one part below the mounds (G1) and the other one lateral and above them (G2). G1 has a wavy and planar cross-bedding and the topmost surface shows abundant horizontal burrows (d). This surface represents an interval of conden-sation and shows a sharp contact (e) with an overlying layer of echinoid-oyster floatstone (30 cm thick) . This layer may have acted as a stable,hard substrate for the settlement of the mounds. G2 is characterized by high angle (22°) trough cross-bedded ooidal grainstones. The mounds are enclosed within these grainstones and interfinger laterally. In the contact region with the mound, crystals of calcite pseudomorphs after gypsum occur in the ooidal shoals. The crystals show concave boundaries with the surrounding sediment (f). These concave boundaries indicate that the sediment was not fullylithified at the time when the gypsum precipitated.

0.5 cm

500 µm2 mm

f g

hIn the field complex interfingering relationships between the mounds and shoals have been documented (a, b). However the studied pol-ished slabs (h) and thin-sections (f, g) clearly show a sharp and ero-sional contact between both carbonate bodies. Sometimes fragments of the microbialites are incorporated in the ooid shoal (white arrows in Fig. h). Ooids are not trapped or bound within the microbialite fabric, al-though occasionally they have been recognized in some of the cavities of the microbialite.These observations point out that both carbonate bodies were not growing simultaneously. First, the mounds established upon a condensed surface and after during the deposition of the shoals,they became truncated and covered by the shoals.

Mineral structures composed of calcite pseudomorphs after gypsum and hematite pseudomorphs after framboidal pyrite were found inside the thrombolites. The occurrence of these minerals is mainly restricted to the growth framework cavities.

The studied mounds exhibit high-relief domal growth morphologies (a) of approx. 1.3 m height and 2.5 m width (n=28). This represents a preferential lat-eral growth direction. The mean space between individual mounds is ~ 2.7 mThe microbialites have a macroscopic clotted fabric and no internal lamina-tion, consequently they are classified as thrombolites (Aitken, 1967). The de-tailed analysis of 25 polished slabs revealed that the thrombolites mainly de-velop branching growth forms (c, d). These branches are 0. 5 - 2 mm thick and radially arranged, showing mainly horizontal growth. In the interspace between the branches, irregular shaped growth framework cavities were formed. Occa-sionally, the mounds are connected laterally, bridgelike with pendant nodular hemispheroids growing downward in the interspace (b). Thehemispheroids are approx. 25 cm thick and 50 cm in diameter.

Different growth stages can be identified, expressed by the occurence of a conti-nous band with abundant encrusters such as bivalves and bryozoans (e, f). During these intervals bivalves frequently bore the outer surface of the micro-bialites (red circles in Fig. d). The occurence and development of the downward growing hemispheroids is related to low to zero sedimentation rates (Leinfelder et al.,1993). Furthermore, this peculiar growth forms have been frequently documented in Jurassic thrombolites (Leinfelder and Schmid, 2000).

Small grains of iron oxides appear dispersed throughout the whole thrombolite, often form-ing bands that rim the growth framework cavi-ties (a). These dark red bands are made byrounded mineral aggregates (b).

1. Alternation with shoalsThe mounds occur on top and embedded with ooidal shoals. The mounds developed directly upon a condensed surface with horizontal burrows, which occur at the top of the underlying shoal. A sharp contact between this surface and the mounds is observed. Moreover moundsare eroded by the shoals, indicating they were not coeval.

2. MorphologyThe mounds developed in subtidal shallow water environments. The domal forms of ~ 1.3 m height with preferential lateral growth indicate certain limitation in the available accommoda-tion space. The pioneer settlement of these mounds is represented by a basal layer with ahigh accumulation of echinoids and oysters.

3. Internal structure The resulting clotted peloidal fabric of the thrombolithes was formed in situ by bacterial calci-fication, whereas trapping and binding of allochtonous grains were insignificant.

4. Mineral structureThe presence of former gypsum crystals is probably related to arid periods. In such periods higher restricted conditions with evaporation rates exceeding precipitation rates would have occurred. Framboidal pyrite is linked to S-reducing bacterial that decay organic matter in ananaerobic environment or microenvironment.

5. Biogenic structureSubpoligonal patterns are identified within the thrombolites. Filamentous (5-15 µm long) and coccoidal bacterial forms (1 – 3 µm diameter) are observed within these subpoligons. They probably represent fossilized bacterial remains.

6. SummaryAll these data indicate that mounds are not coeval with the ooid shoals. Their alternance is probably related to the interplay of climatic changes (arid vs. non-arid periods) and sea-levelfluctuations.

The mesoclots are composed of 60 - 200 µm wide dark, micritic pel-oids (f), which are surrounded by sparry calcite (e). The peloidsoccur isolated or as densly packed clusters.

500 µm

125 µm

250 µm 125 µm

500 µm 500 µm

500 µm

CaDo

CaDo

a b

c d

e f

c

d

S FeCaMg

Ca

SiFe

FeO

0 2 4 6 8 10 12 14 16 18 20keVSkalenbereich 2015 cts Cursor: 0.000

e

10µm

10µm

10µm

10µm O

S

Ca

Fe

f

1 mm

a

500 µm

b

Ca 1mm 1mm S

Sr 20µm S 20µm

cSO

Sr

0 2 4 6 8 10 12 14 16 18 20keVSkalenbereich 1310 cts Cursor: 0.000

d

SEM examinations of polished and slightly etched thrombolite rock chips, reveal the presence of subpolygonal, honeycomb-like patterns (g, h). Individual subpolygonal to rounded pits (5 – 10 μm in diameter) are separated by distinct walls with a flaky shape. Inside the pits or dis-tributet in the meshwork spherical, egg-like masses (1 – 3 μm) occur (i). The shape and size of these spheres fits well to those of known modern bacteria and therefore they are interpreted as fossilized bacterial coc-coids. Also twisted filamentous, rod-like bacteria forms ( 5 - 25 µm long) appear coalesced inside of the subpolygonal pattern (j).

Kazmierczak et al. (1996) interpreted similar structures with abundant spheroids and filaments from the Jurassic as remnants of calcified coccoid cyanobacterial mats. 125 µm2 mm

e f

c

1 cm

When observed with the SEM the rounded aggregates show a framboidal texture, which is exlusively formed by pyrite (Farrand, 1970). These spheroids are 5 - 15 µm in diameter (d) and occur in clusters of 4 or more specimens, often near cavities (c). The elemental composition, obtained with the EDX, reveal that the fram-boids were later oxidized into hematite (e). Despite the smallamounts of S (< 1%) measured in the framboid (f).

500 µm20 µm

a b

+

The calcite pseudomorphs after gypsum are lenticular (~4mm long) and have been found exclusively inside the growth framework cavities of the thrombolite (a, b) and in the shoal deposits in close contact with the mounds. It is obvious, that these crystals precipitated inside the cavi-ties, since the crystals often adapt to the shape of the cavities (a). The gypsum (CaSO4 2 H2O) has been almost completely replaced by calcite; just some minor sulfur inclusions arepresent in the crystals (c).

The small sulfur-rich inclusions consist of celestite (SrSO4), which commonly occur asso-ciated with gypsum (d).

For the study of the microbial mounds two locations have been selected: one, East Island Wall(E-IW), is ~70 m long and the other one, North Island Face (N-IF), has a length of 90 m. The studied deposits belong to the same stratigraphic intervaland can be correlated with each other in both localities. The stratigraphic succession startswith a coral-microbial reef, which is followed by bioclastic wackestones/floatstones and marly deposits. Overlying the marls a package of ooidal-peloidal grainstones were deposited. Enclosed within this interval the microbial mounds occur.

i j

g h

5 µm

5 µm 5 µm

10 µm

J. D. AITKEN (1967): Classification and environmental significance of cryptalgal limestones and dolomites, with illustrations from the Cambrian and Ordovician of southwestern Alberta, Journal of Sedimentary Petrology, V. 37, p. 1163-1178

R. LEINFELDER, M. NOSE, D. SCHMID, W. WEGNER (1993): Microbial Crusts of the Late Jurassic: Composition, Palaeoecological Significance and Importance in Reef Construction, Facies, V. 29, p. 195-230

R. LEINFELDER, D. SCHMID (2000): Mesozoic Reefal Thrombolites and Other Microbolites, Microbial Sediments, Springer-Verlag Berlin Heidelberg, R. Riding and S. Awramik (Eds.), p. 289-294 J. KAZMIERCZAK, M.COLEMAN, M. GRUSZCZYNSKI, S. KEMPE (1996): Cyanobacterial key to the genesis of micritic and peloidal limestones in ancient seas, Acta Palaeontologica Polonica, V. 41, p. 319-338

M. FARRAND (1970): Framboidal Sulphides Precipitated Synthetically, Mineral. Deposita, V. 5, p. 237--247